Preterm delivery represents a prevalent cause of perinatal mortality in the developed world and occurs in approximately 7% to 10% of all deliveries (Berkowitz et al. Epidemiol. Rev. 15:414-443 (1993)). Severe morbidity, especially respiratory distress syndrome, intraventricular hemorrhage, bronchopulmonary dysplasia, and necrotizing enterocolitis, are far more common in preterm than in term infants. Long-term impairment, such as cerebral palsy, visual impairment, and hearing loss, are also more common in preterm infants. At present, preterm birth remains a leading cause of infant mortality and morbidity in the United States, where, despite the significant improvements in obstetrical medicine, the infant mortality rate is higher than in many other industrialized nations, causing costs exceeding $5 billion per year for neonatal intensive care of low birth-weight babies. The actual costs associated with this care are even higher when taking into consideration the healthcare provision of preterm childbirth-related ailments, such as respiratory distress syndrome, heart conditions, cerebral palsy, epilepsy, and severe learning disabilities.
During the past 40 years of clinical investigations, and despite the use of multiple therapeutic agents, the rate of preterm birth has not drastically declined. The prevention of preterm labor is difficult and although tocolytic therapy remains the cornerstone of management of preterm labor, there is not universal agreement as to its value in this condition. The available tocolytic agents on their own do not prolong labor for more than 48 hours, and the majority of these agents lack uterine selectivity and can thus cause potentially serious side effects both for the mother and the fetus.
Fundamentally, term and preterm labor are similar processes in that they share a common physiological endpoint characterized by uterine contractions, cervical dilatation, and activation of the fetal membranes. The differences lie in the gestational age at which these processes occur and the mechanisms by which they are activated. Term labor is thought to result from physiological activation of the terminal pathway, whereas preterm labor is a pathological condition characterized by multiple etiologies in which one or more components of this pathway are aberrantly activated.
Uterine contractility is stimulated or inhibited by various receptors in myometrial cells. It is hypothesized that activation of the myometrium results from the coordinated expression of contraction-associated proteins (CAPs), including actin, myosin, connexin-43, and the receptors for oxytocin and prostaglandins. In general, receptors that provoke calcium entry or calcium release from intracellular stores stimulate contractility. However, receptors coupled to the production of cyclic nucleotides, such as cyclic adenosine monophosphate (cAMP) relax the uterus. For instance, oxytocin and prostaglandin F (FP) receptors are stimulatory, while β2 adrenoceptors and prostaglandin E2 receptors coupled to cAMP formation are inhibitory.
In uterine tissues, prostaglandins E2 (PGE2) and F2α (PGF2α) have been shown to induce cervical changes and elicit uterine contractility, two key events in the physiology of labor and parturition. Activation of the FP receptor in the human myometrium by PGF2α results in the elevation of intracellular calcium concentration, which, in turn, leads to contraction of the uterine smooth cell muscle (Abramovitz et al. J. Biol. Chem. 269:2632-2636 (1994) and Senior et al. Br. J. Pharmacol. 108:501-506 (1993)). FP receptors are up-regulated in uterine tissues towards term (Al-Matubsi et al. Biol. Reprod. 65:1029-1037 (2001)). Inhibitors of prostaglandin synthesis (such as indomethacin and nimesulide) have shown some tocolytic effect but are not devoid of side effects and their un-licensed use in the clinic has raised concerns regarding fetal safety (Norton et al. New Engl. J. Med. 329:1602-1067 (1993) and Peruzzi et al. New Engl. J. Med. 354:1615 (1999)). There remains a need to develop therapeutics with myometrial selectivity that permit lasting inhibition of uterine contractions that lead to labor and that prolong pregnancy to a stage where increased fetal maturation raises the chances of survival.